This invention generally relates to pressure ratio control in two stage compressor assemblies. More particularly, this invention relates to a modulation technique including a mechanism between an output of a second stage of the compressor assembly and an interstage circuit of the assembly to allow fluid flow as needed for pressure ratio modulation.
Two-stage compressor assemblies are known and used for various purposes. Like other compressor designs, two-stage systems have operating temperature limitations dictated by the capability of coatings, materials and acceptable amounts of distortion. In two-stage assemblies, there is the further possible complication that the discharge temperatures of each stage are not balanced throughout the system operating envelope. This imbalance is mainly attributed to a difference in the pressure ratio, efficiency, inlet air temperature, etc., between the two stages.
When the discharge temperatures of the two-stages of the compressor assembly are not balanced, one stage typically operates at a higher temperature than the other and becomes the limiting factor in the ambient capability of the overall compressor assembly. Accordingly, the capability of the compressor to operate at high ambient temperature levels becomes limited as it is dictated by the stage operating at the higher temperature.
One cause of imbalanced discharge temperatures is when the compressor assembly is located above a certain altitude. For a given system operating pressure, the pressure ratio of the second stage and its discharge temperature increases with an increase in altitude. This phenomenon applies from sea level to the maximum operating altitude of the compressor. It follows that above certain altitudes, the capability of the equipment to operate at higher ambient temperatures becomes limited because of the higher second stage pressure ratio.
There is a benefit in balancing the discharge temperatures of each stage in a two-stage compressor assembly. Without such balancing, unacceptably high temperatures can occur during capacity modulation. One conventional technique used in an attempt to avoid unacceptable high temperatures during capacity control includes loading or unloading the compressor at certain intervals. Typical applications of this technique include either fully opening or nearly closing the compressor assembly inlet valve to maximize or effectively cut off the fluid flow into the assembly. During such cycling (i.e., suddenly loading or unloading the compressor), the compressor assembly components are exposed to load and thermal fluctuations that may adversely affect the system reliability.
There is a benefit in implementing a mechanism which permits modulation of the pressure ratio in two-stage oil free compressor assemblies to achieve more balanced discharge temperatures. The benefits consist of increased ambient temperature and altitude capability as well as providing capacity modulation capability. This invention provides this benefit while avoiding the shortcomings and drawbacks of the prior art.
In general terms, this invention is a two-stage compressor assembly having pressure ratio modulation useful for balancing discharge temperatures of each stage. This invention applies to various types of oil free compressors, such as rotary, screw, centrifugal, scroll, and piston, for example.
This invention provides enhanced ambient temperature and altitude capability. At the same time, the inventive arrangement provides capacity modulation capability.
An assembly designed according to this invention includes a first stage compressor portion. A second stage compressor portion is fluidly coupled with the first stage compressor portion downstream from the first stage. An interstage circuit is fluidly coupled between the first and second stage portions so that an output from the first stage portion is upstream of the interstage circuit and the second stage compressor portion is downstream from the interstage circuit. A pressure modulation mechanism selectively couples an output from the second stage portion with the interstage circuit. By selectively allowing fluid flow through the pressure modulation mechanism, the pressure ratio of the second stage can be controlled and kept within desired limits.
In one example, a needle valve is part of the pressure modulation mechanism. The needle valve is controlled to allow a desired amount of flow from the output of the second stage portion back to the interstage circuit. In one example, the bypass mechanism couples the output of the second stage compressor portion to the upstream side of the interstage cooler. In another example, the output from the second stage portion is taken downstream from an after cooler, which is downstream from the second stage portion, and introduced downstream of the interstage cooler by the pressure modulation mechanism.
In one example, an electronic controller controls operation of the flow through the pressure modulation mechanism. The controller controls operation of a valve associated with the pressure modulation mechanism so that the valve is gradually opened when a main inlet valve to the compressor assembly is closing. Similarly, the controller gradually closes the pressure modulation mechanism valve when the main inlet valve is opening.